The present application claims priority to Australian Patent Application No. PS0511, filed Feb. 13, 2002, which application is incorporated herein fully by this reference.
The present invention relates to a method of producing continuous orthogonal signals and a method of their use for detecting changes in a body, and in particular, but not exclusively, to a method of using said signals in reflection seismology.
Reservoir engineers and oil production engineers are interested in the changes that take place in the subsurface when oil or gas is being extracted from reservoirs. The knowledge of changes which are taking place can assist in maximising production from reservoirs and for managing the extraction.
To satisfy this need for information, techniques have been developed for re-surveying a reservoir using seismic techniques whilst attempting to reproduce the original survey. However inherent limitations militate against exact repetition of a survey. Accordingly, re-surveying with current equipment is difficult and the results are affected by uncontrolled and uncontrollable global changes that can obscure local changes in the subsurface which are of economic interest.
For example, a typical surveying method utilises a Vibroseis truck or marine air-gun and an array of geophones which are placed on the ground. The Vibroseis truck produces vibrations which are transmitted through the ground to set up seismic waves. These waves travel through the subsurface and are reflected at seismic boundaries where there is a change in rock properties. The boundaries are often termed as xe2x80x9creflectorsxe2x80x9d. The travel time to these reflectors is measured from the reflected signal. Provided that a velocity model, or the rock type in the subsurface is known, it is possible to determine the distance to a reflector and so build up a model of the subterranean rock structure.
In order to re-survey the area in question, to build up a picture of the changes which may occur, for example during the extraction of oil, it will be necessary to ensure that the Vibroseis or air-gun is positioned at the same location for each survey, that the mechanical vibrations produced are the same between surveys and that geophones for picking up reflected seismic waves are in the same position between surveys and further to ensure that other conditions between surveys are identical.
In practice, this is virtually impossible making the results of the re-surveying imprecise.
The present invention was developed with a view to overcoming the above described problems in relation to surveying the subsurface. However, embodiments of the invention are not limited to geological application and can be used for examining, and/or detecting changes in, other bodies such as for non-destructive testing of man-made structures or for use in medical ultrasound or natural seismology.
According to one aspect of the present invention there is provided a method of producing N continuous orthogonal signals including the steps of:
for each of said N signals, summing a plurality of constituent sine waves together, where each of said sine waves has a known and mutually exclusive frequency and, said sine waves have a random or pseudo-random phase, and wherein no two of said N signals includes constituent sine waves of the same frequency.
Preferably each of said constituent sine waves has a prime number integer frequency.
Preferably said method further includes scaling the frequencies of said constituent sine waves to a predetermined bandwidth.
Preferably said method includes forming each of said N signals with an extended repeat length.
Preferably said method includes forming each of said N signals with a different repeat length.
Preferably the phases of each of the component sine waves are arranged such that frequencies in any of the N signals will not have a phase which causes large reinforcements of amplitude at any time within the repeat length of the N signals.
According to a further aspect of the present invention there is provided a method of examining a physical body including the steps of:
forming N continuous orthogonal signals where N is an integer xe2x89xa71 by, for each of said N signals, summing a plurality of constituent sine waves together, where each of said sine waves has a known and mutually exclusive frequency and, said sine waves have a random or pseudo-random phase, and wherein no two of said N signals includes constituent sine waves of the same frequency transmitting said N signals into said physical body;
providing M receivers for receiving said continuous signals including reflections of said continuous signals from one or more reflectors within said physical body and recording said received signals at each of said M receivers;
determining a travel-time to the reflectors for each of said N signals at said M receivers by cross correlating each of said N signals with each of said recorded signals at said M receivers; and, deriving an image of said physical body from said determined travel-times.
In one embodiment, said step of transmitting said N signals includes transmitting each of said N signals from respective separate sources.
Preferably said step of cross correlating includes cross correlating said respective signals using a correlation window of a width greater than one half of the repeat length of the shortest repeat length of said signals and summing said windowed correlations for a length equal to the product of the repeat lengths of said N signals.
Preferably said step of determining travel-time includes, for each recorded signal, windowing said recorded signal, cross correlating said recorded signal with each of said N signals, and summing separate windows of said recorded data.
Alternately, said step of determining travel-time includes at each of said M receivers, windowing the recorded data, summing said recorded data for the repeat length of the Nth signal and cross correlating said summed record with said source signal.
In an alternate embodiment, the step of transmitting said N signals includes summing each of said N signals to form a composite signal and transmitting said composite signal from a single signal source.
In this embodiment, said step of determining travel-time includes, for a particular one of said N signals constituting said composite signal;
summing said received signal at each of said M receivers for a repeat length of said particular one of said N signals; and,
correlating said summed signal with said particular one of said N signals.
According to a further aspect of the present invention there is provided a method of detecting changes in a physical body including the steps of:
forming N continuous orthogonal signals in accordance with the first aspect of the present invention where N is an integer xe2x89xa71 by, for each of said N signals, summing a plurality of constituent sine waves together, where each of said sine waves has a known and mutually exclusive frequency and, said sine waves have a random or pseudo-random chase, and wherein no two of said N signals includes constituent sine waves of the same frequency transmitting said N signals into a physical body;
providing M receivers for receiving said continuous signals including reflections of said continuous signals from one or more reflectors within said physical body and
recording said received continuous signals at said M receivers times T and T+xcex94; and,
deriving M differenced signals by subtracting said recorded signals at time T from said recorded signal at time T+xcex94 to provide an indication of changes in said body, based on said M differenced signals.
Preferably said method includes analysing said differenced signals in amplitude and phase.
Preferably said method includes back propagating said differenced signal to produce an image in time of changes in said body.
Preferably such back propagation can be achieved by phase conjugation of the differenced signal which is equivalent to time reversal.
Alternately, said method includes, providing a velocity field of said body, and back propagating said differenced signal through said velocity model to produce a spatial image of the changes in said physical body.